1. Field of the Invention
The present invention generally relates to a development device used in an image forming apparatus such as a copier, a printer, a facsimile machine, or a multifunction machine capable of at least two of these functions, a process cartridge incorporating the development device, and an image forming apparatus incorporating the development device.
2. Description of the Background Art
In general, electrophotographic image forming apparatuses, such as copiers, printers, facsimile machines, or multifunction devices including at least two of those functions, etc., include a latent image carrier on which an electrostatic latent image is formed and a development device to develop the latent image with developer with either one-component developer consisting essentially of only toner or two-component developer consisting essentially of toner and carrier.
For example, in development devices using one-component developer (i.e., toner), a developer carrier such as a development roller is disposed contactlessly with the latent image carrier, and the development device supplies the developer to the latent image formed on the latent image carrier by causing the developer to hop and form clouds (i.e., toner clouds) on or around the developer carrier. The developer carriers used in development devices using one-component developer typically include two layers of electrodes electrically insulated from each other, namely, an inner electrode and multiple outer electrodes positioned on an outer side of the developer carrier from the inner electrodes. The multiple outer electrodes are arranged at predetermined intervals (a predetermined pitch) in a circumferential direction of the developer carrier. The developer carrier further includes a surface layer overlaying an outer circumferential side of each outer electrode so as to protect the multiple outer electrodes while electrically insulating the multiple outer electrodes from each other.
In order to form toner clouds using such a developer carrier, the development device further includes a power source for applying separate voltages that change differently from each other with time to the inner electrode and the outer electrodes, respectively, thus generating electrical fields that change differently from each other with time between adjacent outer electrodes. The electrical fields cause the toner carried on the developer to hop between the adjacent outer electrodes and form toner clouds. It is to be noted that the phenomenon of the electrical fields being generated between the adjacent two of the multiple outer electrodes that causes toner to hop, thus forming toner clouds, is hereinafter referred to as “flare” or a “flare state”. In other words, the term “flare” means a phenomenon in which toner hopping on a circumferential surface of the developer carrier forms toner clouds in an adjacent area of the circumferential surface of the developer carrier.
In this type of development device, if the electrical fields are extremely small, toner can neither hop on the developer carrier properly nor form toner clouds because the strength of the electrical fields is weaker than force of adhesion between the toner and the developer carrier. Accordingly, toner is not transferred to the latent image carrier from the developer carrier that is not in contact with the latent image carrier, resulting in a decrease in image density of output images. By contrast, if the electrical fields are extremely large, it is possible that voltage leaks between the inner electrode and each outer electrode, which can damage the electrodes themselves. Moreover, it is possible that voltage leaks between the outer electrodes and the surface layer of the developer carrier overlaying the outer electrodes, thus damaging the surface layer.
Therefore, the size or strength of the electrical fields is a critical factor and must be adjusted properly.
For example, JP-2009-36929-A discloses a development device that maintains a constant electrical potential on the surface of a flare roller, serving as the developer carrier, that includes an inner electrodes and multiple outer electrodes so as to prevent unevenness in the image density and scattering of toner in the backgrounds of output images. This known development device further includes a developer regulator, such as a doctor blade, that regulates the thickness of a toner layer formed on the flare roller and a voltage application device for applying a bias voltage to the developer regulator. The mean value of the bias applied to the developer regulator has an electrical potential identical to the mean value of the bias applied to the multiple outer electrodes of the flare roller.
Although effective for keeping the electrical potential on the surface of the flare roller constant, this known configuration is insufficient for keeping the flare state constant because only the bias voltage applied to the flare roller is considered in this known configuration. More specifically, the flare state also fluctuates due to deviations in the thickness of the surface layer (i.e., insulation layer or protection layer) of the flare roller, which is not considered in this known configuration. The thickness of the surface layer of the developer carrier varies originally due to manufacturing tolerances, and accordingly there are deviations in the proper electrical fields to be generated by the developer carrier. In other words, the electrical field for causing a desired flare state is unique to each developer carrier. Further, the surface layer of the developer carrier is abraded and becomes thinner over time by the contact with the developer regulator and the like, which causes the proper electrical fields for attaining the desired flare state to fluctuate as well.
In view of the foregoing, the inventors of the present invention recognize that there is a need for a development device capable of maintaining a constant flare state around the developer carrier, a process cartridge including the development device, and an image forming apparatus including the development device.